Engines may be configured with various fuel injection systems for delivering a desired amount of fuel to an engine for combustion. One type of fuel injection system includes a port fuel injector which delivers fuel into an intake port of an engine cylinder. Fuel is delivered to the port fuel injector via a port injection fuel rail that is pressurized via a lift pump. Another type of fuel injection system includes a direct fuel injector which delivers fuel directly into an engine cylinder at a higher pressure than the port injector. Fuel is drawn from a fuel tank via the lift pump and then delivered to the direct fuel injector via a direct injection fuel rail that is pressurized via a high pressure pump.
Port and direct fuel injectors are configured to have a dynamic range of fuel injection capabilities. As a result, a single port fuel injector may provide a high fuel injection quantity for maximum cylinder air charge during high engine torque demand conditions as well a small fuel injection quantity for minimum cylinder air charge during low engine torque demand conditions. However, as the fuel injection quantity decreases, the ability of a fuel injector to accurately deliver the desired volume decreases. Specifically, the fuel quantity injected as a “percent of value” may have reduced accuracy as the fuel quantity or pulse width decreases. Fuel air ratio error is proportional to “percent of value” error. Thus, fuel injection errors can result in air-fuel ratio discrepancies in cylinders, leading to misfires, reduced fuel economy, increased tailpipe emissions, and an overall decrease in engine efficiency.
One example approach for increasing the accuracy of delivering small volumes of fuel is shown by Ulrey et al in US20160153383. Therein, a lift pump is intermittently operated to maintain the pressure at an inlet of the higher pressure fuel pump, and at the fuel rails, above fuel vapor pressure. In particular, the lift pump is maintained disabled until a peak outlet pressure of the fuel pump decreases from a peak outlet pressure corresponding to a previous fuel injection pulse. The duration is learned as a minimum pulse duration, and during subsequent low load conditions, a fuel injection pulse having the minimum pulse duration is applied to the fuel pump. However, operating a fuel injector at minimum pulse width may increase fuel consumption due to increased air charge delivery. In addition, fuel vapor purge may be limited due to low fuel injection quantity since vapor purge is typically limited to a function (e.g., 40%) of the entire fuel mass needed for combustion. Enabling the fuel vapor purge to meet emissions standards (for example, to remove approximately 80% of the vapor from the canister with a defined duration of a drive cycle) with the limited vapor purge rate may lead to the need for expensive fuel vapor purge design alternatives (such as a bigger canister or multiple canisters). As such, this may unnecessarily increase component costs. The inventors herein have recognized that port fuel injection may be more fuel vapor tolerant than expected. As a result, port fuel injection accuracy may increase when operated at or around (e.g., slightly above fuel vapor pressure, such as 30 kPa above fuel vapor pressure) because the vapor pressure is substantially constant and free of fuel injection-caused pressure pulsations. Therefore the issues described above may be at least partly addressed by a method for an engine comprising: in response to a drop in engine load, deactivating a lift pump; and port injecting fuel while fuel rail pressure remains at or around fuel vapor pressure, with the lift pump deactivated. In this way, low fuel mass port injection accuracy can be improved while extending a duration that a lift pump is disabled.
As one example, in response to a drop in engine load (e.g. when torque demand is low), the lift pump may be deactivated and the lift pump deactivation may be maintained while the fuel rail pressure decreases from a first rail pressure all the way to (or near to) a fuel vapor pressure. Port fuel injection to combusting cylinders of the engine may be continued while the fuel rail pressure decreases from the first rail pressure all the way to (or near to) the fuel vapor pressure. Port injection may be further continued while fuel rail pressure remains at fuel vapor pressure, with the lift pump deactivated, for a duration. Over the duration, an amount of fuel injected by the injectors may be accumulated. Once the accumulated fuel amount reaches a threshold (e.g., 10% of the fuel rail volume), the lift pump may be reactivated to re-pressurize the fuel rail. Thereafter port injection may be continued with the lift pump on. This mode may be precluded if the vehicle is significantly off level (e.g., at greater than 3° tilt) as measured by the vehicle's inertial reference (or a tilt sensor). This reduces the necessity of testing this mode in off-angle positioning.
In this way, the on-duration of a fuel lift pump may be reduced. As a result, energy consumption of a fuel pump may be minimized without causing fuel vapor ingestion issues at the fuel rail. By reducing fuel rail pressure to a vapor pressure that that is at or around fuel vapor pressure (e.g., 30 kPa above fuel vapor pressure) for a limited duration of time, while a lift pump is disabled, a small quantity of liquid fuel instead of a combination of liquid fuel and vaporous fuel may be accurately injected into the engine cylinders up to a threshold volume without ingesting fuel vapor. In addition, the injector-to-injector variability and shot-to-shot variability of a given injector may be reduced, which allows for cost reduction in the fuel vapor handling system. Further, the need to operate port fuel injectors at minimum pulse width is obviated. This reduces the amount of air charge delivered to engine cylinders at low loads, leading to lower fuel consumption, and fewer cylinder-to-cylinder air/fuel ratio and torque deviations. Furthermore, fuel vapor purging is not limited, increasing canister purging efficiency over a given drive cycle.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.